1. Field of the Invention
The present invention relates generally to a ball screw and more particularly, to a ball screw capable of real-time surveillance and control of preload.
2. Description of the Related Art
A ball screw is a common component in precision positioning and keeps rolling to serve as the power transmission interface between the screw rod and the screw nut for much reduction of frictional resistance in the process of movement.
The conventional double-nut ball screw includes two screw nuts, a preload piece mounted between the two nuts, two keyways formed on respective tonic surfaces of the two nuts and axially facing each other, and a link having two ends located inside the two keyways, respectively. In this way, while the screw rod is running, the two screw nuts keep combined with each other with a preload keeping oppressed onto the preload piece to further avoid potential axial gap resulting from movement of the ball screw and to enhance the rigidity, positioning precision, and positioning stability of the ball screw in operation. For this reason, the preload has become one of essential indexes of measuring the rigidity, positioning precision, and positioning stability of the ball screw.
However, after the ball screw has been used for a period of time, wear and tear happens among the balls, the screw rod, and the screw nuts to result in axial gap leading to decrement of preload between the two screw nuts in such a way that the rigidity, positioning precision, and positioning stability of the ball screw will become worse.
Taiwan Patent Publication No. 319129 disclosed a piezoelectric type pre-stress adjustment and position fine tuning mechanism of a ball screw, in which two portioning rings are mounted between two screw nuts, a pre-compressed spring holder and a pre-compressed spring are mounted between the partitioning rings and the screw nuts, and a piezoelectric actuator is mounted between the partitioning rings; a piezoelectric actuator and a dynamometer are mounted inside the piezoelectric actuator. However, such mechanism of this patent is structurally very complicated and it is not easy to assembly the mechanism. Besides, the variation of the preload between the two screw nuts cannot be directly measured but needs to be measured through the partitioning rings or other components, so the measurement result is not accurate.
Taiwan Patent Publication No. 201204960 disclosed a method diagnostic of preload ineffectiveness of a ball screw and a device based on the method, in which a voiceprint signal generated while the ball screw is working can be filtered by empirical mode decomposition (EMD), then processed by Hilbert-Huang transform (HHT) to generate Hilbert-Huang spectrum (HHS), next processed by multi-scale entropy extraction to generate multi-scale entropy complexity mode, and after the raw multi-scale entropy complexity mode and the current multi-scale entropy complexity mode are compared, whether the preload of the ball screw disappears or not can be effectively diagnosed. However, in the process of measurement based on such patent, the measuring accuracy is subject to the vibration, noise, or frequency generated during the processing. Therefore, such prior art still fails to provide accurate measuring outcome.